Optical apparatus, tracking apparatus and optical disk apparatus

ABSTRACT

An optical apparatus can eliminate offset resulting from assembling errors and also can correct offset even in the photodetector of any optical system. An optical disk apparatus for reading information from tracks traces the track with the condensed spot of light beam depending on the tracking error signal obtained from the optical system for tracking error detection. The tracking error detecting optical system includes an optical apparatus having a photodetector for outputting, as the tracking error signal, a differential value of outputs of a plurality of light receiving sections for receiving the light beam condensed by the condenser lens. Also, an optical element is provided for relatively increasing and/or decreasing the amount of receiving light of a light receiving section depending on displacement of the optical axis of the light beam, in order to balance the light received by the light receiving sections and compensate for affect.

[0001] The present invention relates to an optical apparatus including aphotodetector having a plurality of light receiving sections to receivea condensed light beam, a tracking apparatus including the opticalapparatus, and a disk apparatus including the optical apparatus. Thepresent invention particularly relates to an optical apparatus whichcompensates for offset resulting from displacement of an optical axis ofa condensed light beam with respect to a photodetector.

BACKGROUND OF THE INVENTION

[0002] A photodetector for receiving a condensed light beam is generallydesigned as a two-divided detector or a four-divided detector. Thetwo-divided detector has a light receiving section which is divided intoa couple of light receiving regions to receive a condensed spot of acondensed light beam. The four-divided detector has a light receivingsection divided into four light receiving regions. The two-divideddetector is arranged in such a manner that a spot of a condensed beamequally illuminates both light receiving regions. Moreover, thefour-divided detector is arranged in such a manner that a spot of acondensed beam illuminates equally the four light receiving regions.

[0003] These two-divided detectors and four-divided detectors are usedin an optical disk apparatus. The two-divided detector is typically usedas a detector for tracking error detection and as a detector for readinginformation. The four-divided detector is typically used as a detectorfor focus error detection.

[0004]FIG. 1 shows an optical system of an optical disk apparatus. Theoptical system shown in FIG. 1 includes a detector for tracking errordetection and a detector for focus error detection. The light beamemitted from a light source 7 becomes a coherent light beam through acollimator lens 12. The coherent light beam passes a beam splitter andis then condensed by an objective lens 20. The condensed light beamilluminates a disk 9. The objective lens 20 forms a light beam spot onthe disk 9.

[0005] The disk 9 is all optical disk on which many information tracksor other targets are formed. In FIG. 1, the optical system structured byincluding the collimator lens 12, the beam splitter 13, and theobjective lens 20 is called a light condensing optical system 8. Thelight beam condensed on a recording surface of the disk 9 is alsoreflected by the recording surface. Moreover, the light beam isdiffracted depending on the information recorded on the disk 9. Thereflected light beam is converted to a parallel beam passing through theobjective lens 20. Thereafter, the reflected light beam is bent in itslight path by 90 degrees with the beam splitter 13. The light beam ofwhich the light path direction has been changed is then passed into abeam splitter 14.

[0006] This coherent light beam is split, by this beam splitter 14, to alight beam directed to the optical system 10 for focus error detectionand a light beam directed to the optical system 11 for tracking errordetection. The optical system 10 for focus error detection comprises afour-divided photodetector 5, an optical system 10 a introducing anasymmetrical spot shape of the condensed light beam, and a condenserlens 10 b for condensing the light beam on the four-dividedphotodetector 5. The optical system 11 for tracking error detectioncomprises a two-divided photodetector 6 and a condenser lens 11 a forcondensing the light beam on this two-divided photodetector.

[0007] The light beam which is deflected by the beam splitter 13 isdivided into one light beam input to a reproducing optical system (notillustrated) for reproducing information recorded on the disk 9, andanother light beam entered into the beam splitter 14.

[0008] In the optical system 11 for tracking error detection, deviationbetween the information track formed on the disk 9 and the beam spotformed on the disk 9 by the objective lens 20 is detected. As thedetecting method, various methods are known. The most popular methodutilizes the diffraction phenomenon generated on the recording surfaceof the disk 9. In a rewritable magneto-optical disk, a guide groove isformed between adjacent tracks. This groove is arranged as many groovesformed in the equal interval in the track width having constant width.Many grooves enables tile disk surface to work as the diffractiongrating. When a beam spot having a diameter almost equal to the trackwidth is focused on the disk surface and is then reflected therefrom,the mirror-surface reflected light beam and diffracted light beam aregenerated. The diffracted light beam interferes with the light elementreflected by the mirror-surface to largely change the distribution oflight beam strength reflected from the disk 9. The beam could also bedirected through the disk 9 if the disk were transparent.

[0009]FIG. 2 shows the distribution of reflected beam strengthinfluenced by the diffracted light beam. FIG. 2(a) shows thedistribution of strength when the beam spot is located at the center ofthe track. FIG. 2(b) shows the distribution of strength when the beamspot is located at the position deviated by about ¼ the width of thetrack from the center of the track. Distribution of the light beamstrength is largely different at the portions where the mirror-surfacereflected light beam and diffracted light beam are overlapped or notoverlapped. In this case, the reflected light beam is divided into aleft side region and a right side region with a straight line (brokenline A in the figure) which passes the center of the reflected lightbeam and is parallel to the track. The beam strength in the left sideregion of the straight line A is compared with that in the right sideregion. In the case of FIG. 2(a), the beam strength in the left sideregion is equal to that in the right side region. Meanwhile, in the caseof FIG. 2(b), since the diffracted beam is unbalanced, a certaindifference is generated in tile strength of the left side and right sideregions. This difference in strength is detected by the two-dividedphotodetector having the dividing light parallel to the track on thedisk 9. Namely, the difference of output of the two-dividedphotodetector 6 corresponds to the difference of beam strength and alsoto the amount of deviation of the spot from the center of the track.Therefore, such amount of deviation is detected in higher accuracy. Thismethod is called a push-pull method.

[0010] This push-pull method detects the amount of deviation revealed bythe difference of strength of the two-divided photodetector. Therefore,when the beam spot center is deviated from the dividing line of thetwo-divided photodetector, offset is generated in the differentialoutput of the two-divided photodetector.

[0011]FIG. 3 shows the condition where deviation of the optical axisoccurs. In FIG. 3, a chain line indicates the main beam 4 a of the lightbeam when the optical axis 20 a of the objective lens is aligned withthe optical axis of the light beam 4. When the objective lens 20 movesupward (direction of arrow mark B) and is located at the positionindicated by a broken line, the main beam of the light beam is input tothe objective lens 20, but the main beam of the light beam is deviatedfrom the optical axis 20 a of the objective lens 20. The light beam isrefracted by the objective lens 20 and is then directed to the disk 9.The light beam is reflected by the disk 9 and is then input to theobjective lens 20. The optical axis 4 b of the main beam of the incidentlight beam of the objective lens 20 is deviated upward (direction ofarrow mark B) from the optical axis 20 a of the objective lens 20. Thisreflected light beam is indicated by a broken line.

[0012] Thereafter, the light beam passes the beam splitters 13, 14 andis then input to the optical system 11 for tracking error detection.Next, this light beam is input to the condenser lens 11 a at theposition where the main beam thereof is deviated downward (direction ofarrow mark C) for the optical axis of the condenser lens 11 a of theoptical system 11 for tracking error detection. The main beam input tothe condenser lens 11 a is refracted by the condenser lens 11 a and isthen input after it is deviated downward (direction of arrow mark C)from the photodetector 6.

[0013] The entire part of the light beam reflected by the disk 9 isdeviated downward of the photodetector 6 around the optical axis 4 b ofthis deviated main beam. As explained above, if the optical axis of themain beam of the light beam is deflected from the dividing line of thetwo-divided photodetector, deviation is generated between thesymmetrical line of the strength distribution and the dividing line ofthe two-divided photodetector. Therefore, a differential output of thetwo-divided photodetector includes offset. This differential output iscalled as a push-pull signal.

[0014]FIG. 4 shows a push-pull signal. The horizontal axis of FIG. 4indicates the position in the direction crossing the track, while thevertical axis of FIG. 4 indicates level of the push-pull signal.

[0015] In FIG. 4, the push-pull signal (a) indicates the push-pullsignal in such a case that there is no deviation between the opticalaxis of the objective lens and that of the light beam input to theobjective lens, while the push-pull signal (b) indicates the push-pullsignal in such a case that there is a deviation of 200 μm between theoptical axis of the objective lens and that of the optical beam input tothe objective lens. The push-pull signal (b) corresponds to the casewhere the objective lens is deviated from the aperture diameter by about5%. The aperture diameter of the objective lens is 3.3 mm. The trackwidth is 1.1 μm.

[0016] The push-pull signal (a) corresponds to the push-pull signalwhere the condensed spot of light beam has moved, with no displacementof the optical axis, to the groove in the opposite side crossing thetrack from the one groove of the track. If there is no displacement ofthe optical axis, the push-pull signal becomes zero when the condensedspot is located at the center of the track. When an optical head orobjective lens 20 is moved to make the push-pull signal zero, thecondensed spot can be accurately located to the center of the track.Moreover, the signal waveform of the push-pull signal is perfectlysymmetrical in the positive and negative sides of the coordinates 0, soit is the ideal control signal.

[0017] The push-pull signal (b) corresponds to the push-pull signal asthe condensed spot of light beam moves to the groove in the oppositeside crossing the track from the one groove of the track when there is adisplacement of the optical axis. In this case, if the condensed spot islocated at the center of the track, the push-pull signal does not becomezero. When location of the condensed spot is controlled to make thepush-pull signal zero, the spot is located to the position deviated fromthe center of the track. The push-pull signal (b) of FIG. 4 includes thedeviation of about 0.1 μm as the offset. This displacement of theoptical axis generated when the objective lens moves can be generated byassembling errors of the condenser lens.

[0018] In general, when information is read from a disk, positionaldeviation between a condensed spot and a track center is generated dueto eccentricity of the disk. The positional deviation between thecondensed spot and the track center can be corrected when the objectivelens 20 moves in the direction crossing orthogonal to the trackdirection. Since such positional deviation is often generated, movementof the objective lens is also generated frequently and thereby opticalaxis displacement of the optical beam also occurs frequently. If theoptical axis center of the objective lens is deviated by the distance dfrom the optical axis of the light beam, the optical axis of thereflected beam is deviated by 2d from the optical axis of the objectivelens.

[0019] If the disk 9 deflects the optical axis of the light beam by anangle Θ, when the focal distance of the objective lens is defined as f,the amount of displacement can be expressed as 2fΘ. In addition, whenthe track density on the disk becomes higher, the degree of influence byoffset due to the optical axis deviation and degree of influence byassembling errors becomes larger.

[0020] An offset of the tracking error signal will interfere with thedata read/write operation. A typical optical disk apparatus is providedwith an optical disk medium having a recording surface in which thewidth of the information track is about 1 μm. Data is recorded alongthis information track. In order to read this data, the condensed spotof the light beam must be accurately positioned on this informationtrack. The width of the recording pits formed at the center position inthe width direction of the track is narrower than the width of thetrack.

[0021] When a condensed spot is deviated too far from the center of thetrack during a data reading operation, (i) data can no longer be readbecause the condensed spot does not overlap on the recording mark, or(ii) data reading accuracy and speed are lowered because the data signallevel becomes lower than the noise signal level.

[0022] When a disk is exchangeable for a disk drive and data is writtenunder the condition that track deviation is generated within the diskdrive, the data written in such a disk cannot be read in some cases withdifferent disk drives.

[0023] Displacement of the optical axis during a condensed spot positionadjustment can be avoided by conducting a condensed spot positionadjustment by moving the entire part of the optical pickup including acondensing optical system and optical system for tracking errordetection. In this case, since the optical pickup as a whole, which isheavier than the optical system for tracking error detection, is moved,the maximum response rate of a condensed spot position adjustingoperation is limited.

[0024] Moreover, an offset included in a push-pull signal because ofdeviation between an optical axis of an objective lens and that of alight beam may be canceled by adding a bias signal to the push-pullsignal depending on a signal indicating the amount of movement of anobjective lens from a position detector which detects the position ofthe objective lens. In this case, the position detector and a biassignal adding circuit must additionally be provided.

[0025] In addition, as shown in Japanese Laid-Open Patent ApplicationNo. SHO 59-38939, there is a method in which an offset compensatingsignal is produced from a specific shape of pits formed on the track ofthe disk. In this case, the specific shape of pits must be formed on thedisk and thereby the disk format must be changed.

[0026] Moreover, as shown in Japanese Laid-Open Patent Application No.HEI 8-306057, there is a method in which an offset compensating signalcan be produced using a particular arithmetic formula from manyreceiving outputs from the photodetector in which the light receivingsection is divided into many regions. In this case, a circuit forconducting the arithmetic operation indicated by the particulararithmetic formula must be provided.

[0027] In the methods explained above, offset resulting from thecondenser lens used in the optical system for tracking error detectionand assembling error of the photodetector cannot be corrected. Inaddition, the methods explained above are proposed to eliminate offsetincluded in the differential output of the photodetector in the opticalsystem for tracking error detection, and these methods cannot be appliedas a method of correcting offset of the photodetector of the otheroptical system, namely, of the optical system for focus error detectionand optical system for information reading.

OBJECTS OF THE INVENTION

[0028] It is an object of the present invention to provide a new andimproved optical apparatus which can eliminate negative effects ofoffset resulting from assembling errors.

[0029] It is another object of the present invention to provide a newand improved optical apparatus which can correct for offset of a lightbeam or photodetector in any type of optical system.

[0030] Still another object of the present invention to provide a newand improved optical apparatus which can easily and economicallyeliminate the effects of offset without any additional detector andaddition of any particular circuit.

[0031] Yet another object of the present invention is to provide a newand improved optical apparatus which can move at high speed.

[0032] It is a further object of the present invention to provide a newand improved tracking apparatus which can realize a high precisiontracking operation.

[0033] It is a still further object of the present invention to providea new and improved optical disk apparatus which can realize a highprecision information reading operation.

SUMMARY OF THE INVENTION

[0034] In keeping with one aspect of the invention, an optical apparatushas a photodetector including a plurality of light receiving sections toreceive a light beam condensed by a condenser lens. An optical elementis provided for relatively increasing the amount of receiving light ofone light receiving section and relatively limiting or reducing theamount of receiving light of another light receiving section, dependingon displacement of the optical axis of the light beam.

[0035] In another aspect of the invention, a tracking apparatus isprovided for tracking the condensed spot of light beam on a track orother target on the basis of a tracking error signal obtained from anoptical system for tracking error detection. The optical system fortracking error detection includes an optical apparatus having aphotodetector which outputs, as a tracking error signal, a differencevalue outputs of a plurality of light receiving sections for receivingthe light beam condensed by the condenser lens. It also has an opticaldevice for relatively increasing the amount of light received by onelight receiving section, which relatively reduces the amount ofreceiving light of another light receiving section, depending ondisplacement of the optical axis of the light beam.

[0036] Moreover, an optical disk apparatus for reading information froma track while tracking the spot of light beam on the track on the basisof a tracking error signal obtained from an optical system for trackingerror detection, can include an optical apparatus having a photodetectorand an optical element which compensates for off-track conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] These and other features of this invention and the manner ofobtaining them will become more apparent, and the invention itself willbe best understood by reference to the following description of anembodiment of the invention taken in conjunction with the accompanyingdrawings, in which:

[0038]FIG. 1 is a diagram showing an optical system of an optical diskapparatus;

[0039] FIGS. 2(a) and 2(b) are diagrams showing the intensitydistribution of a reflected beam influenced by a diffracted beam;

[0040]FIG. 3 is a diagram showing the condition that optical axisdisplacement is generated;

[0041]FIG. 4 is a diagram representing a push-pull signal;

[0042]FIG. 5 is a diagram showing a structure of an optical diskapparatus to which the present invention is applied;

[0043]FIG. 6 is a diagram showing a detail structure of portions of theapparatus of FIG. 5;

[0044] FIGS. 7(a) and 7(b) are diagrams for explaining a firstembodiment of the present invention;

[0045]FIG. 8 is a diagram showing an optical element in which the innerregion thereof is formed of a couple of semi-circular regions;

[0046]FIG. 9 is a diagram showing an optical element in which the innerregion thereof is formed with an elliptical shape;

[0047]FIG. 10 is a diagram showing the condition that the optical axisof the light beam input to the optical element is deflected from theoptical axis of the optical element;

[0048]FIG. 11 is a diagram showing the amount of a light beam passingthe regions A to D of the optical element of FIG. 10;

[0049]FIG. 12 is a diagram showing change of the push-pull signalgenerated by the optical element of FIG. 10;

[0050]FIG. 13 is a diagram showing the optical system of the opticaldisk apparatus including the optical element shown in FIG. 7;

[0051] FIGS. 14(a) and 14(b) are diagrams showing another example of theoptical element of the present invention;

[0052] FIGS. 15(a) and 15(b) are diagrams showing another example of theoptical element of the present invention;

[0053] FIGS. 16(a) and 16(b) are diagrams showing yet another example ofthe optical element of the present invention;

[0054] FIGS. 17(a) and 17(b) are diagrams showing still another exampleof the optical element of the present invention;

[0055] FIGS. 18(a) and 18(b) are diagrams showing another example of theoptical element of the present invention;

[0056]FIG. 19 is a diagram showing a still further example of theoptical disk apparatus of the present invention;

[0057] FIGS. 20(a) and 20(b) are diagrams showing another example of theoptical apparatus of the present invention;

[0058]FIG. 21 is a diagram showing a structure of optical disk apparatuswith the optical apparatus shown in FIG. 20;

[0059] FIGS. 22(a) and 22(b) are diagrams showing another example of theoptical element of the present invention;

[0060]FIG. 23 is a diagram showing a structure of an optical diskapparatus with the optical element shown in FIGS. 22(a) and 22(b);

[0061]FIG. 24 is a diagram showing another example of the opticalelement of the present invention;

[0062]FIG. 25 is a diagram showing still another example of the opticalelement of the present invention;

[0063]FIG. 26 is a diagram showing yet another example of the opticalelement of the present invention; and

[0064]FIG. 27 is a diagram showing a practical example of the opticalapparatus shown in FIGS. 22(a) and 22(b).

DETAILED DESCRIPTION

[0065] A preferred embodiment of the present invention will be explainedwith reference to the accompanying drawings. The attached drawings areincluded as a part of the specification to form a part thereof in orderto explain the principles of the invention in combination with thedescription of the specification.

[0066]FIG. 5 shows a structure of an optical disk apparatus to which thepresent invention is applied. In FIG. 5, a magneto-optical disk 100 isdriven to rotate by a spindle motor (not illustrated). Thismagneto-optical disk 100 allows formation of many tracks. Between thetracks, a groove is formed.

[0067] The magneto-optical disk 100 can be a medium loaded to a 3.5 inchmagneto-optical disk cartridge conforming to the ISO format or any othersuitable medium. Information recorded on the magneto-optical disk 100 isread using an isolated optical system 102 and a fixed optical system104. The isolated optical system 102 is structured by including acarriage 108 which is guided by two guide rails 106 a, 106 b andmagnetic circuits 110 a, 110 b which make reciprocal movement on thecarriage 108. The carriage 108 is provided with an optical pickupsection 112 which has an objective lens. The fixed optical system 104 isstructured by including the condensing optical system, tracking errordetecting optical system, focus error detecting optical system andinformation reading optical system.

[0068]FIG. 6 shows detail structures of the isolated optical system andfixed optical system. In FIG. 6, only an objective lens 114 and atriangular prism 116 in the isolated optical system 102 are shown. Amagnetic circuit to drive the objective lens 114 in the directions ofarrow marks A, B is not illustrated. The part enclosed by a broken lineis included in the fixed optical system.

[0069] A light beam emitted from a semiconductor laser 118 is convertedto a parallel beam by a collimator lens 120. This parallel beam is inputto a triangular prism 116 via a beam splitter 122. The light beamreflected from the magneto-optical disk 100 is directed to the beamsplitter 122 via the objective lens 114 and triangular prism 116.

[0070] The light beam could also be diffracted through a transparentdisk, if desired, and then directed to appropriate optical processingapparatus.

[0071] The beam splitter 122 directs the light path of this reflectedbeam to a beam splitter 124. The condensing optical system is structuredby including the collimator lens 120, beam splitter 122, triangularprism 116 and objective lens 114.

[0072] The light beam input to the beam splitter 124 is divided into acouple of light beams directed to a Wollaston prism 126 and condenserlens 128, respectively. The light beam directed to the Wollaston prism126 is condensed by a condenser lens 130. The condensed spot of thelight beam is focused on a two-divided photodetector 132. Thistwo-divided photodetector 132 detects a magneto-optical signal. Themagneto-optical signal MO is obtained as a differential output (H-I) ofa couple of light receiving sections H, I of the two-dividedphotodetector 132. The information reading optical system is structuredby including the Wollaston prism 126, condenser lens 133 and two-dividedphotodetector 132.

[0073] The light beam input to the condenser lens 128 is condensed bythe condenser lens 128 and is divided into a couple of light beams by abeam splitter 134. One light beam passes the beam splitter 134 and iscondensed on a two-divided photodetector 136. This two-dividedphotodetector 136 outputs a tracking error signal. The tracking errorsignal TE is obtained as a differential output (E-F) of a couple oflight receiving sections E, F of the two-divided photodetector 136. Thetracking error detecting optical system is structured by including thecondenser lens 128, beam splitter 134 and two-divided photodetector 136.

[0074] The other light beam passes the beam splitter 134 and a couple ofglass sheets 138 a, 138 b and is then focused to a four-dividedphotodetector 140. These two sheets of glass plate 138 a, 138 b areoptical systems for introducing an asymmetrical property to the lightbeam. The four-divided photodetector 140 outputs a focus error signal.The focus error signal FE is obtained on the basis of outputs of fourlight receiving sections A, B, C, D of the four-divided photodetector140. Namely, the focus error signal FE is obtained by the arithmeticexpression (A−B)+(C−D). The focus error detecting optical system isstructured by including the condenser lens 128, beam splitter 134, twosheets of glass plate 138 a, 138 b and four-divided photodetector 140.

[0075] Such a magneto-optical disk drive is disclosed in JapaneseLaid-Open Patent Application No. TOKKAI HEI 5-151753.

[0076] FIGS. 7(a) and 7(b) illustrate a first embodiment of the presentinvention. A two-divided photodetector 136 detects a tracking errorusing the push-pull method. In other words, the photodetector 135 has aplurality of light receiving sections, each of which can receive atleast a portion of the encoded light beam for purposes of aligning thelight beam with a disk track or other target.

[0077] An optical element 145 is arranged between the two-dividedphotodetector 136 and condenser lens 128 (FIG. 6). The optical element145 balances the distribution of light in the light receiving sectionsof the photodetector 136 if the optical axis of the light beam isdisplaced with respect to the light receiving sections when the lightbeam is aligned with the center of a track on a disk medium or othertarget.

[0078] The optical element 145 is arranged between the condenser lensand the light receiving section. The optical element 145 may be providedbetween the condenser lens 128 and the beam splitter 134, or attachedbetween the light beam emitting surface of the beam splitter 134 and thephotodetector 136. The optical element 145 is provided with adiffraction grating or other light directing device.

[0079]FIG. 7(a) shows an optical system viewed from the beam incidentdirection. FIG. 7(b) shows a light path transmitting through the opticalsystem viewed from the lateral direction. The light receiving section ofthe two-divided photodetector 136 is divided into two sub-sections bymeans of a dividing line K. The two-divided photodetector 136 isprovided with a first light receiving section 136 a and a second lightreceiving section 136 b. An optical axis of the two-dividedphotodetector 136 is set on the dividing line and at the center in thewidth direction of the two-divided photodetector 136.

[0080] The optical system 145 is divided into a couple of regions with aline L parallel to the dividing line. The optical system 145 is providedwith a circular region in which the light beam travels in straight.

[0081] This circular region has a diameter which is equal to or a littlesmaller than the beam diameter of the light beam. The radius of thislight beam is preferably 475 μm at the point where the light beam entersthe optical system 145. Meanwhile, the radius of this circular region isset to 460 μm.

[0082] This circular region includes a couple of regions 145 a, 145 bdivided by the dividing line. These two regions 145 a, 145 b are notprovided with diffraction gratings. Therefore, the light beam input tothis circular region travels in straight to radiate the two-dividedphotodetector 136. The regions 145 c, 145 d at the outside of thecircular regions 145 a, 145 b of the optical system 145 are respectivelyprovided with diffraction gratings. As shown in FIG. 7(b), thediffraction grating of the region 145 c causes a part of the light beaminput to the region 145 c to radiate on the light receiving section 136b of the two-divided photodetector 136. The diffraction grating of theregion 145 d causes a part of the light beam input to the region 145 dto radiate on the other light receiving section 136 a of the two-dividedphotodetector 136.

[0083] Here, the push-pull signal DPP of the two-divided photodetector136 can be obtained by the following arithmetic expression. In thisarithmetic expression, each region of the optical system 145 is giventhe codes A to D and the amount of light input to each region A to D isexpressed as IA, IB, IC, ID. $\begin{matrix}{{{DPP}\left( {d,K} \right)} = \frac{\left( {{IA} + {ID}} \right) - \left( {{IB} + {IC}} \right)}{{IA} + {IB} + {IC} + {ID}}} & (1)\end{matrix}$

[0084] Since the amount of light IA to ID changes depending on theamount of the displacement d of the condensed spot of light beamcondensed by the objective lens 114 from the track center and the amountof axial displacement K of the light beam input to the optical element145, the push-pull signal DPP is expressed as the function DPP(d,K) ofthese two parameters d, K.

[0085] Here, the radius of the circular region formed of the regions 145a, 145 b is set to satisfy the following expressions for a certainpredetermined value K.

DPP (0, K)=0   (2)

DPP (dmax, K)=−DPP (dmin, K)   (3)

[0086] A value dmax is the amount of displacement of the condensed spotfrom the track center when the push-pull signal becomes maximum.Meanwhile, dmin is the amount of track deviation of the condensed spotfrom the track center when the push-pull signal becomes minimum. Formula(2) indicates the condition that the value of the push-pull signalbecomes 0 when the spot is located at the center of the track. Formula(3) indicates the condition where the absolute value of the push-pullsignal when the push-pull signal becomes maximum, is equal to theabsolute value of the push-pull signal when the push-pull signal becomesminimum. A value of the radius of this circular region indicates theboundary for discriminating the light beam transmitting regions 145 a145 b and light beam diffracting regions 145 c, 145 d.

[0087] The radius of the circle defining the regions can be obtained byexecuting the arithmetic operations of the condition formulae (2), (3)using the radius as a parameter. As the value of this radius, theoptimum value satisfying the conditions of formulae (2), (3) isemployed. In FIG. 7, the circular regions 145 a, 145 b of the opticalelement 145 are formed as true circles.

[0088]FIG. 8 is a diagram showing an optical element in which thecircular regions thereof are formed of a couple of arcs. In FIG. 8, theoptical element 155 has a region for transmission of a light beamconsisting of a first region 155 a and a second region 155 b. Theoptical element 155 has a region for diffraction of the light beamconsisting of a first region 155 c and a second region 155 d. This firstregion 155 c and second region 155 d are partitioned by the line Lparallel to the track direction. The first and second regions 155 a, 155b are circular regions in radius r. The center 155 a 1 of the firstcircular region 155 a is displaced by the distance d from the center 155z of the optical axis of the optical element 155. The center 155 b 1 ofthe second circular region 155 b is displaced by the distance d from thecenter 155 z of the optical axis of the optical element 155. Thedisplacing direction of the distance d crosses tie dividing line Lorthogonally. When such an optical element 155 is used, since there aretwo factors of freedom of the radius r and the eccentric position d asthe design parameters, the design values (values of the radius andeccentric position d) satisfying the condition formulae call surely beobtained.

[0089]FIG. 9 is a diagram showing an optical element in which thecircular region of the optical system is formed in an elliptical shape.In FIG. 9, the optical element 165 has the region for transmission ofthe light beam formed as the elliptical region 165 a indicated by a longaxis a and a short axis b. The optical element 165 has the light beamdiffracting region consisting of the first region 165 c and the secondregion 165 d. The first region 165 c and second region 165 d arepartitioned by the line L parallel to the track direction. The centerposition 165 a 1 of the elliptical region 165 a is matched with thecenter of the optical axis 165 z of the optical element 165. As will beapparent from FIG. 9, the long axis a of the elliptical region 165 aextends in the direction parallel to the dividing line L, namely, in thetrack direction. The short axis b extends in the direction orthogonal tothe dividing line L, namely, in the direction across the track. WhenSuch an optical element 165 is used, there are two parameters of thelong axis a and short axis b as the design parameters, and therefore itis possible to obtain the value which uniquely satisfies the conditionformulae.

[0090]FIG. 10 is a diagram showing the condition that the optical axisof the light beam input to the optical element is deviated from theoptical axis of the optical system. This displacement of the opticalaxis of the light beam is generated due to movement of the objectivelens 114 or assembling error of the condenser lens 128, etc. Thegeneration of optical axis displacement is explained in regard to FIGS.1 to 3.

[0091] The optical system shown in FIG. 10 uses the optical element 145as shown in FIG. 7. The light beam 150 a indicated by a dotted line issuch that the optical axis of the light beam 150 a matches with theoptical axis of the optical element 145. Namely, the optical beam 150 ais matched with the circular regions 145 a, 145 b of the optical element145. Since the light beam 150 a is not diffracted when passing throughthe circular regions 145 a, 145 b, the light beam 150 a passing thecircular region 145 a totally illuminates the light receiving section136 a of the two-divided photodetector 136, while the light beam 150 apassing the circular region 145 b totally illuminates the lightreceiving section 136 b of the two-divided photodetector 136. In thiscase, the amounts of receiving light of both the light receiving section136 a and the light receiving section 136 b of the two-dividedphotodetector 136 is equal. Therefore, there is no relative differencein the amount of receiving light of the light receiving sections 136 a,136 b.

[0092] Meanwhile, the light beam 150 b indicated by a solid line isdeflected, in its optical axis of the light beam 150 b, in the directionof arrow mark A for the optical axis of the optical element 145. In thesame manner, the condensed light spot 150 h focused on the two-dividedphotodetector 136 is deviated to the side of the light receiving section136 a of the two-divided photodetector 136. The light receiving section136 a of the two-divided photodetector 136 receives the condensed lightspot 150 h in the amount larger than that of the light receiving section136 b. On the contrary, with respect to the amount of receiving lightonly related to the condensed light spot 150 h, the light receivingsection 136 a receives a relatively larger amount than that of the lightreceiving section 136 b.

[0093] Here, the term “relatively” refers to a comparison based on theamount of receiving light when the light beam 150 b passing only withinthe range of the circular regions 145 a, 145 b of the optical element145 is received by the light receiving sections 136 a, 136 b of thetwo-divided photodetector 136.

[0094] Here, a part 150 b 1 of the light beam 150 b is diffracted by thediffracting region 145 c of the optical element 145. This diffractedlight illuminates, as the condensed spot 150 i in the crescent shape,the light receiving section 136 b of the two-divided photodetector 136.Since the light beam 150 b is not diffracted by the diffracting region145 d of the optical element 145, the diffracted light does notilluminate the light receiving section 136 a of the two-dividedphotodetector 136. Since the light receiving section 136 b receives bothcondensed spot 150 h and the crescent-shaped condensed spot 150 i,imbalance in the amount of receiving light between the light receivingsections 136 a and 136 b is reduced. As explained above, the amount ofthe light beam passing the circular regions 145 a, 145 b received by thelight receiving section 136 b is relatively lower than the amount oflight beam passing the circular regions 145 a, 145 b received by thelight receiving section 136 a. On the other hand, since the light beamdoes not pass the diffraction grating 145 d, the amount of receivinglight of the light receiving section 136 a does not increase, but sincethe light beam passes the diffraction grating 145 c, the amount ofreceiving light of the light receiving section 136 b increases.

[0095] Namely, the diffraction grating 145 c relatively increases theamount of receiving light of the light receiving section 136 b, whichwould otherwise be relatively reduced depending on the optical axisdisplacement of the light beam, in comparison with the amount of thelight beam of the light receiving section 136 a in which the amount ofreceiving light would increase relatively, depending on the optical axisdisplacement of the light beam. The amount of receiving light in thelight receiving section 136 b would otherwise be relatively reduced incomparison with amount of receiving light of the light receiving section136 a. Namely, the light not received by the other light receivingsection 136 a due to the optical axis displacement of the light beam ispolarized by the optical element 145 and is then input to the lightreceiving section 136 b in which the amount of receiving light wouldotherwise be reduced due to the optical axis displacement of the lightbeam.

[0096] Here, “relatively” is used because comparison is made for theamount of receiving light when the light beam 150 b passing only theregions of the diffraction grating regions 145 c, 145 d of the opticalelement 145 is received by the light receiving sections 136 a, 136 b ofthe two-divided photodetector 136.

[0097] In FIG. 10, as an example of imbalance in the amount of receivinglight of the light receiving sections 136 a and 136 b, movement of theobjective lens 20 in parallel to the disk 9 is explained. Axialdisplacement of the light beam input to the two-divided photodetector136 is also generated when the assembling position of a condenser lensis deviated. Moreover, such axial displacement is also generated whenthe disk 9 is inclined against the objective lens 20. If these axialdisplacements are generated, the light beam condensed by the condenserlens is indicated as the light beam 150 b for the optical element 145 asshown in FIG. 10. Therefore, imbalance in the amount of receiving lightresulting from these axial displacements may be corrected by the opticalelement 145 provided between the condenser lens and two-dividedphotodetector 136.

[0098] Moreover, similar to an optical system for focus error detectionand an optical system for information leading, imbalance in the amountof receiving light of the two-divided photodetector and four-dividedphotodetector is also generated if there is deviation of the assemblingposition of the condenser lens and inclination of the disk. Suchimbalance in the amount of receiving light can tie corrected byintroducing an optical element 145 to these optical systems.

[0099]FIG. 11 is a diagram showing the amount of receiving light passingthe regions A to D of the optical element. In FIG. 11 a change in theamount of incident light is indicated when the condensed spot is movedfrom track to track when the optical axis of the objective lens 114 isdeviated by 200 μm from the optical axis of the condensing opticalsystem in the direction crossing the track. When the condensed spot islocated at the track center and the optical axis of the objective lens114 is deflected by 200 μm, the incident light beam of the opticalelement 145 and two-divided photodetector 136 exists at the positionindicated by a solid line in FIG. 10. The amount of light IA indicatesthe amount of light in the region A. The amount of light IB indicatesthe amount of light in the region B. The amount of light IC indicatesthe amount of light in the region C, and the amount of light IDindicates the amount of light in the region D.

[0100] As shown in FIG. 11, a difference is generated between the amountof light IA in the region A and the amount of light IB in the region B,due to axial displacement of the objective lens 114. Since the trackingservo control is conducted in such a manner that the push-pull signal isto be zero, the amounts of light IA and IB are controlled to be equal.The point where the absolute values of the amounts of light IA and IBare matched with each other is shifted in the positive direction fromthe position where the track offset value is zero.

[0101] A value in the amount of light IC of the diffraction gratingregion C changes depending on relative movement of the condensed spotand track. The value obtained by adding the amount of light IC to theamount of light IB is indicated as the amount of light IB+IC. A value ofthe amount of light ID of the diffraction grating region D increasesafter a part of the light beam 150 b has reached the position passingthe diffraction grating region 145 d. In FIG. 11, the amount of light IDis zero. The amount of light IC is given, as explained using FIG. 10, tothe light receiving section 136 b to which the amount of light IB isgiven. Therefore, the amount of incident light of the light receivingsection 136 b becomes equal to the amount of light (IB+IC) obtained byadding the amount of light IB and amount of light IC. Since the amountof light IC is input to the detector in the opposite side, imbalancebetween a couple of light receiving sections can be corrected. In FIG.11, when the track offset value is 0, a total amount of receiving lightof the two light receiving sections 136 a, 136 b is equal. Namely,offset of the push-pull signal generated by shift of the light beam canbe eliminated by providing the optical element 145. This means that thecondition in formula (2) is satisfied.

[0102]FIG. 12 shows change of the push-pull signal. The waveform A isthe push-pull signal when deviation of the objective lens is 0 μm. Thewaveform B is the push-pull signal when deviation of the objective lensis 100 μm. The waveform C is the push-pull signal when deviation of theobjective lens is 200 μm. The waveform D is the push-pull signal whendeviation of the objective lens is 300 μm.

[0103] A differential value of the amount of light IA and the amount oflight (IB+IC) is the push-pull signal. Namely, the push-pull signal TEcan be expressed as IA−(IB+IC). As is apparent from FIG. 12, thepush-pull signal changes only the amplitude depending on change of theaxial displacement of the objective lens, and keeps the symmetricalwaveform around the zero point. This property of the waveform of thepush-pull signal in the present invention maintains linearity at thearea near the track center, and enables generation of a stable controlsignal.

[0104] In this embodiment, the two-divided photodetector, which is thesame as that used in the ordinary push-pull method, is used as thephotodetector to improve the spot position signal. Therefore, anexisting detecting system including the arithmetic circuit may beapplied easily.

[0105] In addition, as the reference of the track center of thepush-pull signal, the spot position where the push-pull signal becomes 0is used. Meanwhile, the present invention allows the application of themethod in which the reference point of the push-pull signal at the trackcenter is set at the intermediate point between the peak and bottompoints of the push-pull signal.

[0106] When a method is employed in which the reference point of thetrack center is set to the intermediate point between the peak andbottom points of the push-pull signal, the influence of stray light canbe alleviated. This is because the optical system satisfies thecondition of formula (3). Since the stray light exists, if light ofconstant intensity is input to one input of the two-dividedphotodetector, unexpected bias is generated in the push-pull signal.This bias can be eliminated by setting the center point between the peakand bottom of the push-pull signal as the reference of the track center.Thereby, since symmetry in the positive and negative sides of thepush-pull signal near the track center is maintained, good linearity ofthe push-pull signal can be maintained at the area near the trackcenter, to assure stable control.

[0107]FIG. 13 shows an optical system of an optical disk apparatus whichhas the optical element shown in FIG. 7. In FIG. 13, the structuralelements which are the same as those in FIG. 1 are designated by thesame reference numerals and the same explanation is omitted.

[0108] In FIG. 13, an optical element 145 is provided within the opticalsystem for tracking error detection. The optical system 11 for trackingerror detection is structured by including a two-divided photodetector6, an optical element 145 and a condenser lens 11 a. The two-dividedphotodetector 6 and optical element 145 correspond to an opticalapparatus. Moreover, a differential value of the two-dividedphotodetector 6 is input to a tracking control section as the trackingerror signal. The tracking control section drives, depending on theinput tracking error signal, a tracking coil which reciprocally movesthe objective lens 20 in parallel to the surface of disk 9. The trackingcoil is driven to cause the tracking error signal to be zero so that thecondensed is always located at the track center. The tracking errordetecting optical system 11 and tracking control section correspond to atracking apparatus.

[0109] The optical element 145 is arranged between the two-dividedphotodetector 6 and the condenser lens 11 a. As explained above, theoptical element 145 has it region, in its center area, which is equal toor smaller than the light beam diameter, and transmits the input lightbeam. The optical element 145 is provided, at the external region ofthis dividing line parallel to the dividing line of the two-dividedphotodetector 6, and is designed to cause the light beam input to suchsubsections to be incident to a photodetecting portion in the oppositeside of the dividing line.

[0110] When the optical axis of the objective lens 20 is not deflectedfrom the optical axis of the condensing optical system 8, the light beamcondensed by the condenser lens 11 a passes only the center region ofthe optical element 145. Meanwhile, when the optical axis of theobjective lens 20 shifts in the direction of arrow mark A, the lightbeam condensed by the condenser lens 11 a is deviated in the directionof arrow mark B. The amount of light passing the center region of theoptical element 145 received by the light receiving section 6 bincreases in comparison with the amount of light passing the centerregion of the optical element 145 received by the light receivingsection 6 a. Simultaneously, a part of the light beam condensed by thecondenser lens is diffracted by the diffracting region 145 c of theoptical element 145 and is then input to the light receiving section 6 aof the two-divided photodetector 6. Therefore, the amount of receivinglight of the light receiving section 6 a increases to correct imbalancein the amounts of receiving light of the light receiving sections 6 aand 6 b.

[0111]FIG. 14 shows another example of the optical element. FIG. 14(a)shows a light path of light passing the optical element viewed from thebeam incident direction. FIG. 14(b) shows a light path of light passingthe optical system viewed from the lateral direction. In FIGS. 14(a) and14(b), an optical element 175 is provided with circular regions 175 a,175 b in which the light beam travels in straight. This circular regionhas a diameter which is equal to or a little smaller than the light beamdiameter. This circular region includes a couple of regions 175 a, 175 bdivided by the dividing line. These two regions 175 a, 175 b are notprovided with diffraction grating and are formed as only the glass platesuch as a transparent flat plate. Therefore, the light beam input to thecircular region travels in straight to radiate the two-dividedphotodetector 6. The regions 175 c, 175 d at the outside of the circularregions 175 a, 175 b of the optical element 175 are provided withprisms. The diffraction grating becomes a factor of error at the time ofcompensating for imbalance in the amount of light because the amount oflight diffracted changes depending on the diffraction efficiency. Whenthe incident light is refracted by the prism, the incident light can betotally input to the photodetector in the opposite side, so compensationaccuracy is high.

[0112]FIG. 15 shows another example of the optical element. FIG. 15(a)shows the optical element viewed from the beam incident direction. FIG.15(b) shows the light path of the light beam passing the optical elementviewed in the lateral direction. In FIGS. 15(a) and 15(b), the opticalelement 185 is provided with circular lens regions 185 a, 185 b to whichthe light beam is condensed. This circular region has a diameter whichis equal to or a little smaller than the light beam diameter. Thiscircular region includes a couple of regions 185 a, 185 b divided by thedividing line. These two regions 185 a, 185 b are not provided with adiffraction grating. The light beam input to this circular region isfurther condensed by the circular region to radiate the two-dividedphotodetector 6. Since the condensed spot size of the light beam inputto the photodetector is reduced, the photodetector size can also bereduced. When the area is smaller, the photodetector can assure quickerresponse and thereby accurate track error signal can be obtained. Theregions 185 c, 185 d at the outside of the circular regions 185 a, 185 bof the optical system 185 are provided with a lens. This lens is dividedinto two sections with the dividing line parallel to the dividing lineof the two-divided photodetector, and is designed to cause the incidentlight beam to enter the light receiving section in the opposite side ofthe dividing line. Namely, this optical element 185 is an elementcombining a plurality of lenses. According to this embodiment, theproblem of the diffraction efficiency can be solved as in the case ofthe embodiment shown in FIG. 14.

[0113]FIG. 16 shows another example of the optical element. FIG. 16(a)shows the optical element viewed in the beam incident direction. FIG.16(b) shows the light path of light beam passing the optical systemviewed in the lateral direction. In FIGS. 16(a) and 16(b), the opticalelement 195 is provided, at its center area, with a couple of conicalprisms 195 a, 195 b. The external diameter of this circular prism isequal to or a little smaller than the beam diameter of the light beam.These two regions 195 a, 195 b are not provided with the diffractiongrating. The light beam input to the first conical prism 195 a isrefracted by the conical prism 195 a to radiate the light receivingsection 6 b of the two-divided photodetector arranged in the oppositeside of the dividing line L. The light beam input to the second conicalprism 195 b is refracted by the conical prism 195 b, in the same manner,and is input to the light receiving section 6 a arranged on the oppositeside of the dividing line. The regions 195 c, 195 d at the outside ofthe two conical prisms 195 a, 195 b of the optical element 195 areformed as the flat surface to transmit the light beam. The light beampassing the external side region 195 c enters the light receivingsection 6 a arranged on the same side as the external region 195 c. Thelight beam passing the external region 195 d enters the light receivingsection 6 b arranged on the same side as the external region 195 d.

[0114] When the optical axis of the objective lens 20 is not deflectedfrom the optical axis of the condensing optical system 8, the light beamcondensed by the condenser lens 11 a passes only the center region ofthe optical element 195. The light beam having passed the conical prism195 a of the optical element 195 enters the light receiving section 6 bof the two-divided photodetector 6. The light beam having passed theconical prism 195 b of the optical element 195 enters the lightreceiving section 6 a of the two-divided photodetector 6. When theoptical axis of the light beam condensed by the condenser lens 1 a ismatched with that of the optical element 195, imbalance due to theoptical axis displacement is not generated in amounts of receiving lightof the light receiving sections 6 a and 6 b. Namely, in this case, bothamounts of receiving light arc relatively equal.

[0115] When the optical axis of the objective lens 20 is deflected fromthe optical axis of the condensing optical system 8, the optical axis ofthe light beam condensed by the condenser lens is deflected from theoptical axis of the optical element 195. If the optical axis of thelight beam is assumed to be deflected in the left direction from theoptical axis of the optical element 195, the amount of light passing theconical prism 195 b is reduced and thereby the amount of receiving lightof the light receiving section 6 a is also reduced. The amount ofreceiving light of the light receiving section 6 a is relatively reducedin comparison with the amount of receiving light of the light receivingsection 6 b. Here, since the light beam does not pass the flat plateregion 195 d, the amount of receiving light is never added to the lightreceiving section 6 b. Since a part of the light beam passes the flatplate region 195 c, the amount of the receiving light of a part of thelight beam is added to the light receiving section 6 a. The amount ofreceiving light of the light receiving section 6 a relatively increasesin comparison with the amount of receiving light of the light receivingsection 6 b. Therefore, the amount of receiving light of the lightreceiving section 6 a increases to correct imbalance in the amounts ofreceiving light of the light receiving sections 6 a, 6 b.

[0116] Namely, in this embodiment, the light beam passing the centerregion of the optical element 195 is changed in the direction to bereceived by the light receiving section on the opposite side of thedividing line. Meanwhile, the light passing the external region of theoptical element 195 is never changed in the direction to be received bythe light receiving section on the same side of the dividing line.

[0117]FIG. 17 shows still another example of the optical element. FIG.17(a) shows the optical element viewed from the beam incident direction.FIG. 17(b) shows the optical path, viewed from the lateral direction, ofthe light beam passing the optical element. In FIGS. 17(a) and 17(b),the optical element 205 is provided, at its center area, with a coupleof diffraction gratings 205 a, 205 b. The external diameter of thisdiffraction grating is equal to or a little smaller than the diameter ofthe light beam passing the optical element 205. The light beam input tothe first diffraction grating 205 a is refracted by the firstdiffraction grating 205 a and enters the light receiving section 6 b ofthe two-divided photodetector arranged in the opposite side of thedividing line L. The light beam input to the second diffraction grating205 b is also refracted by the second diffraction grating 205 b andenters the light receiving section 6 a arranged in the opposite side ofthe dividing line L. The external regions 205 c, 205 d of the twodiffraction gratings 205 a, 205 b of the optical element 205 are formedas the flat surfaces allowing the light beam to pass. The light beampassing this external region 205 c also enters the light receivingsection 6 a arranged in the same side as the external region 205 c. Thelight beam passing the external region 205 d enters the light receivingsection 6 b arranged in the same side as the external region 205 d.

[0118] When the optical axis of the objective lens 20 is not alignedwith that of the condensing optical system 8, the light beam condensedby the condenser lens 11 a passes only the center area of the opticalelement 205. The light beam having passed the diffraction grating 205 aof the optical element 205 enters the light receiving section 6 b of thetwo-divided photodetector 6. The light beam having passed thediffraction grating 205 b of the optical element 205 enters the lightreceiving section 6 a of the two-divided photodetector 6. When theoptical axis of light beam condensed by the condenser lens 11 a isaligned with the optical axis of the optical element 205, imbalance byoptical axis displacement is not generated in the amounts of receivinglight of the light receiving sections 6 a and 6 b. Namely, in this casethe amounts of receiving light are relatively equal.

[0119] When the optical axis of the objective lens 20 is deflected fromthe optical axis of the condensing optical system 8, the optical axis ofthe light beam condensed by the condenser lens is deflected from that ofthe optical element 205. If the optical axis of the light beam isdeviated in the left direction from the optical axis of the opticalelement 205, the amount of receiving light passing the diffractiongrating 205 b is reduced and thereby the amount of receiving light ofthe light receiving section 6 a is also reduced. The amount of receivinglight of the light receiving section 6 a is relatively lower incomparison with the amount of receiving light of the light receivingsection 6 b. Here, since the light beam does not pass the flat plateregion 205 d, the amount of receiving light is never added to the lightreceiving section 6 b. Since a part of the light beam passes the flatplate region 205 c, the amount of receiving light of a part of the lightbeam is added to the light receiving section 6 a. The amount ofreceiving light of the light receiving section 6 a relatively increasesin comparison with the amount of receiving light of the light receivingsection 6 b. Therefore, the amount of receiving light of the lightreceiving section 6 a increases to correct imbalance in the amount ofreceiving light of the light receiving sections 6 a, 6 b. Accordingly,track error signal offset can be eliminated.

[0120]FIG. 18 shows yet another example of the optical element. FIG.18(a) shows the optical element viewed from the beam incident direction.FIG. 18(b) shows the light path, viewed from the lateral direction, ofthe light beam passing the optical element. In FIGS. 18(a) and 18 b),the optical element 215 is provided, at its center area, with a firstcircular diffraction grating 215 a. The external diameter of thisdiffraction grating is equal to or a little smaller than the diameter ofthe light beam passing the optical element 215. The two-dividedphotodetector 6 is provided, along the dividing line L, at the positionshifted in the right direction of the figure from the optical axis ofthe optical element 215. The first diffraction grating 215 a changes thedirection of the light path of the light beam, to be input to the firstdiffraction grating 215 a, to enter the two-divided photodetector 6. Theoptical element 215 is provided, at the external region of the firstdiffraction grating 215 a, with the second and third diffractiongratings 215 c, 215 d. The second diffraction grating 215 c changes thedirection of the light path of the light beam passing the seconddiffraction grating 215 c so that this light beam enters the lightreceiving section 6 a arranged in the opposite side of the dividing lineL for the second diffraction grating 215 c. The third diffractiongrating 215 d changes the direction of light path of the light beampassing the third diffraction grating 215 d so that this light beamenters the light receiving section 6 b arranged in the opposite side ofthe dividing line L for the third diffraction grating 215 d.

[0121] When the optical axis of the objective lens 20 is aligned withthe optical axis of the condensing optical system 8, the light beamcondensed by the condenser lens 11 a passes only the center area of theoptical element 215. Since the optical axis of the light beam condensedby the condenser lens 11 a is aligned with the optical axis of theoptical element 215, the light beam having passed the diffractiongrating 215 a of the optical element 215 equally enters the lightreceiving sections 6 a, 6 b of the two-divided photodetector 6. Thelight beam enters the two-divided photodetector 6 so that the opticalaxis of condensed spot formed on the two-divided photodetector 6 islocated oil the dividing line of the two-divided photodetector 6.

[0122] When the optical axis of the objective lens 20 is deflected fromthe optical axis of the condensing optical system 8, the optical axis ofthe light beam condensed by the condenser lens is deviated from that ofthe optical element 215. If the optical axis of the light beam isassumed to be deviated in the lower direction in the figure from theoptical axis of the optical element 215, the condensed spot formed onthe two-divided photodetector 6 is shifted in the lower direction in thefigure. Since the focusing position of the condensed spot is shifted inthe lower direction, the amount of receiving light of the lightreceiving section 6 a is relatively reduced compared with the amount ofthe light receiving section 6 b. Here, the light beam does not pass thethird diffraction grating 215 d and therefore the amount of receivinglight is never added to the light receiving section 6 b. Meanwhile,since a part of the light beam passes the second diffraction grating 215c, the amount of receiving light of a part of this light beam is addedto the light receiving section 6 a. The amount of receiving light of thelight receiving section 6 a relatively increases in comparison with theamount of receiving light of the light receiving section 6 b. Therefore,the amount of receiving light of the light receiving section 6 aincreases to correct imbalance in the amount of receiving light of thelight receiving sections 6 a and 6 b. Accordingly, the track errorsignal offset can be eliminated. According to the present embodiment,the two-divided photodetector may be arranged in the desired position.Considering the arrangement position of the two-divided photodetector,the diffracting direction of the diffraction grating to be provided inthe optical element can be set.

[0123]FIG. 19 shows another example of the optical disk apparatus. InFIG. 19, the light beam emitted from the light source 7 is converted tothe parallel coherent light beam by the collimator lens 12. Thiscoherent light beam passes the beam splitter 13 and is then condensed bythe objective lens 20. The focused light beam illuminates the disk 9.The objective lens 20 forms a light beam spot on the disk 9. T his disk9 is an optical disk on which many information tracks or other targetsare formed. In FIG. 19, the optical system including the collimator lens12, beam splitter 13 and objective lens 20 is called the condensingoptical system 8. The light beam condensed on the recording surface ofthe disk 9 is reflected by the recording surface. Moreover, this lightbeam is diffracted depending on the information recorded on the disk 9.The reflected light beam is recovered to the parallel light when itpasses the objective lens 20. Thereafter, the reflected light beam isbent, in its light path, by 90 degrees by the beam splitter 13. Thelight beam, of which the light path direction is changed, enters theoptical system 10 for focus error detection.

[0124] The focus error detecting optical system 10 includes thefour-divided photodetector 5, optical element 10 a providing anasymmetrical condensed light beam spot, and condenser lens 10 bcondensing the light beam on the four-divided photodetector 5. Theoptical element 10 a is formed by including two sheets of glass plate asshown in FIG. 6. The focus error detecting method is called the Foucaultmethod. As the focus error detecting method, various detecting methodsother than the Foucault method can be applied. For example, as the focuserror detecting method, the astigmatism method using a cylindrical lenscan be applied.

[0125] The light beam, having changed its direction with the beamsplitter 13, is divided by another beam splitter, not illustrated,arranged between the beam splitter 13 and focus error detecting opticalsystem 10, into one light beam entering the reproducing optical system(not illustrated) for reproducing the information recorded on the disk9, and another light beam entering the optical system 10. Thisreproducing optical system is structured by including, as shown in FIG.6, the Wollaston prism 126, condenser lens 130 and two-dividedphotodetector 132 for magneto-optical detection. In this case, the disk9 is a 3.5 inch magneto-optical disk medium conforming to the ISOformat.

[0126] The optical system for tracking error detection to detectdeviation between the light beam spot formed by the objective lens 20and track on the disk 9 is structured by including the condensingoptical system 8 and a package 220. The package 220 includes a lightsource 7 formed of a semiconductor laser, a two-divided photodetector226 for detecting tracking error and an optical element 225. Thetwo-divided photodetector 226 and optical element 225 are structured inthe same manner as the optical element 215 and two-divided photodetector6 as shown in FIG. 18. This optical element 225 is provided, at itscenter area, with the first circular diffraction grating 215 a. Thefirst diffraction grating 215 a causes the light beam passing the firstdiffraction grating 215 a to enter the light receiving sections 6 a, 6 bof the two-divided photodetector 6. When the optical axis of the lightbeam is aligned with the optical axis of the condensing optical system8, the first diffraction grating 215 a causes the diffracted light beamto enter the two-divided photodetector 6 so that the optical axis of thecondensed spot formed on the two-divided photodetector 6 is located onthe dividing line of the two-divided photodetector 6. The opticalelement 215 is provided with the second and third diffraction gratings215 c and 215 d in the external region of the first diffraction grating215 a. The second diffraction grating 215 c changes the direction of thelight path of the light beam passing the second diffraction grating 215c so that the light beam enters the light receiving section 6 a arrangedin the opposite direction of the dividing line L for the seconddiffraction grating 215 c. The third diffraction grating 215 d changesthe direction of the light path of the light beam passing the thirddiffraction grating 215 d so that the light beam enters the lightreceiving section 6 b arranged in the opposite side of the dividing lineL for the third diffraction grating 215 d.

[0127] The light beam emitted from the semiconductor laser 7 travels instraight when it is not diffracted at the time of passing the opticalelement 225. The light beam reflected by the disk 9 is condensed on thetwo-divided photodetector 226 by the collimator lens 12.

[0128] On the magneto-optical disk medium 9, tracks and grooves arealternately formed. The light beam diffracted by the groove on the disk9 enters the two-divided photodetector 226 via the condensing opticalsystem 8. The two-divided photodetector 226 detects, using the knownpush-pull method, deviation between the track center and a beam spotformed on the disk 9.

[0129] According to the present embodiment, the two-dividedphotodetector 226 is not always required to be arranged on the opticalaxis 4 a. Therefore, the light source 7 and two-divided photodetector226 can be arranged on the same plane. Accordingly, the light source 7and two-divided photodetector 226 can be mounted at the bottom surfaceof the package 220, and the electric circuits used for the light source7 and two-divided photodetector 226 can be integrated. In addition,since the beam splitter for polarizing the light beam for tracking errordetection is no longer required, the optical apparatus can be reduced insize and can be formed at a low cost. Moreover, the loss of the amountof light of the light beam does not happen when it passes the beamsplitter. Moreover, since the optical element 225 can be attached to thepackage 220, the structure of the optical system for tracking errordetection can be simplified. In addition, since the light beam used fortrack error detection is the diffracted light beam, when the spacefrequency of all diffraction gratings forming the optical element is setequally, the diffraction efficiency can also be set equally, and therebycalculation for obtaining the amount of receiving light can also besimplified.

[0130]FIG. 20 shows another example of the optical apparatus. FIG. 20(a)shows the optical element viewed from the light beam incident direction.FIG. 20(b) shows the light path of light beam passing the opticalelement viewed from the lateral direction. In FIGS. 20(a) and 20(b), theoptical element 235 is provided, at its center area, with a circulardiffraction grating. The external diameter of this diffraction gratingis equal to or a little smaller than the beam diameter of the light beampassing the optical element 235. This diffraction grating is formed ofthe first and second diffraction gratings 235 a, 235 b. The first andsecond diffraction gratings 235 a, 235 b are divided with the dividingline L which is parallel to the track direction. A couple ofphotodetectors 236, 238 are arranged in symmetrical positions withrespect to the dividing line. The first diffraction grating 235 achanges the direction of the light path of the light beam so that thelight beam to be input to the first diffraction grating 235 a enters thephotodetector 236. The second diffraction grating 235 b changes thedirection of the light path of the light beam so that the light beam tobe input to the second diffraction grating 235 b enters thephotodetector 238. The optical element 235 is provided with the thirdand fourth diffraction gratings 235 c, 235 d at the external regions ofthe first and second diffraction gratings 235 a, 235 b. The thirddiffraction grating 235 c changes the direction of the light path of thelight beam passing the third diffraction grating 235 c so that the lightbeam enters the photodetector 238 arranged in the opposite side of thedividing line L for the third diffraction grating 235 c. The fourthdiffraction grating 235 d changes the direction of the light path of thelight beam passing the fourth diffraction grating 235 d so that thelight beam enters the photodetector 236 arranged in the opposite side ofthe dividing line L (or optical axis 4 a) for the fourth diffractiongrating 235 d.

[0131]FIG. 21 shows the structure of the optical disk apparatus in theoptical apparatus shown in FIG. 20. In FIG. 21, the light beam emittedfrom the light source 7 is converted to the parallel light beam by thecollimator lens 12. This coherent light beam passes the beam splitter 13and is then condensed by the objective lens 20. This condensed lightbeam illuminates the disk 9. The objective lens 20 forms a light beamspot on the disk 9. This disk 9 is an optical disk on which manyinformation tracks or other target are formed. In FIG. 21, the opticalsystem structured by including the collimator lens 12, beam splitter 13and objective lens 20 is called the condensing optical system 8. Thelight beam condensed on the recording surface of the disk 9 is reflectedby the recording surface. Moreover, this light beam is diffracteddepending on the information recorded on the disk 9. The reflected lightbeam is recovered as the coherent light beam when it passes theobjective lens 20. Thereafter, the reflected light beam is bent, in thislight path, by 90 degrees by the beam splitter 13. The light beam ofwhich the light path direction is changed is input to the optical system10 for focus error detection.

[0132] The focus error detecting optical system 10 is structured byincluding a four-divided photodetector 5, an optical element la forgiving an asymmetrical characteristic to the condensed light beam spot,and a condenser lens 10 b for condensing the light beam on thefour-divided photodetector 5. The optical element 10 a is formed of twosheets of glass plate, as shown in FIG. 6.

[0133] The light beam, which is changed in its direction by the beamsplitter 13, is divided into one light beam to enter the reproducingoptical system (not illustrated) to reproduce the information recordedon the disk 9, and another light beam to enter the optical system 10 bythe beam splitter, not illustrated, arranged between the beam splitter13 and the focus error detecting optical system 10.

[0134] The tracking error detecting optical system to detect deviationbetween the light beam spot formed by the objective lens 20 and thetrack on the disk 9 is structured by including the condensing opticalsystem 8 and a package 230. The package 230 includes a light source 7formed of the semiconductor laser, a couple of photodetectors 236, 238for detecting tracking error and an optical element 235. These twophotodetectors 236, 238 and optical element 235 form the opticalapparatus as shown in FIG. 20.

[0135] The light beam emitted from the semiconductor laser 7 travels instraight when it is not diffracted at the time of passing the opticalelement 235. The light beam reflected by the disk 9 passes thecondensing optical system 8 and is then condensed by the collimator lens12. The condensed light beam goes to the package 230.

[0136] The light beam condensed by the collimator lens 12 is diffracted,when it passes the optical element 235, by the first and seconddiffraction gratings 235 a, 235 b and then enters the photodetectors236, 238. A differential output of the two photodetectors 236, 238 isoutput as the tracking error signal indicating deviation between thetrack center and beam spot formed on the disk 9 by the objective lens20.

[0137] When the optical axis of the objective lens 20 is aligned withthe optical axis of the condensing optical system 8, the light beamcondensed by the collimator lens 12 passes only the circular regionformed by the first and second diffraction gratings 235 a, 235 b of theoptical element 235. The light beam having passed the first and seconddiffraction gratings 235 a, 235 b of the optical element 235 enters acouple of photodetectors 236, 238 providing an equal amount of receivinglight.

[0138] When the optical axis of the objective lens 20 is deflected fromthe optical axis of the condensing optical system 8, the optical axis 4a of the light beam condensed by the collimator lens is also deflectedfrom the optical axis of the optical element 235. When it is assumedthat the optical axis 4 a of the light beam is deviated to the lowerdirection in the figure from the optical axis of the optical clement235, the amount of the receiving light beam passing the firstdiffraction grating 235 a is reduced and thereby the amount of receivinglight of the photodetector 236 is also reduced. Namely, the amount ofreceiving light of the photodetector 236 is relatively reduced incomparison with the amount of receiving light of the photodetector 238.Here, since the light beam does not pass the third diffraction grating235 c, the amount of receiving light is never added to the photodetector238. Meanwhile, since a part of the light beam passes the fourthdiffraction grating 235 d, the amount of receiving light of a part ofthe light beam is added to the photodetector 236. The amount ofreceiving light of the photodetector 236 relatively increases incomparison with the amount of receiving light of the photodetector 238.Therefore, the amount of receiving light of the photodetector 236increases, and then imbalance in the amount of receiving light of thephotodetectors 236 and 238 is corrected. Thereby, the track error signaloffset can be eliminated.

[0139] In this embodiment, the effect similar to that of the opticaldisk apparatus shown in FIG. 19 can be obtained. Moreover, since thetwo-divided photodetector of the embodiment shown in FIG. 19 must beprovided to result in alignment of the optical axis of the light beamand the dividing line, the working time becomes longer. On the otherhand, this embodiment can independently provide the two photodetectors236, 238, so it is simplified in its installation work.

[0140]FIG. 22 shows another example of the optical apparatus. FIG. 22(a)shows an optical element viewed from the beam incident direction. FIG.22(b) shows the light path of the light beam passing the opticalelement. In FIGS. 22(a) and 22(b), the optical element 245 is structuredby including five diffraction gratings. The first and second diffractiongratings 245 a, 245 b are divided into a couple of sections by thedividing line L parallel to the track direction. A couple ofphotodetectors 246, 248 are arranged in the line in symmetricalpositions with respect to the dividing line. The first diffractiongrating 245 a changes the direction of the light path of the light beamso that the light beam to be input to the first diffraction grating 245a enters the photodetector 246. The second diffraction grating 245 bchanges the direction of the light path of the light beam so that thelight beam to be input to the second diffraction grating 245 b entersthe photodetector 248. The first and second diffraction gratings 245 a,245 b form the circular region. The first and second diffractiongratings 245 a, 245 b correspond to the first diffraction gratingpreviously described.

[0141] This optical element 245 is provided, at the external regions ofthe first and second diffraction gratings 245 a, 245 b, with the thirdand fourth diffraction gratings 245 c, 245 d. The third diffractiongrating 245 c changes direction of light path of the light beam passingthe third diffraction grating 245 c so that the light beam enters thephotodetector 248 arranged in the opposite side of the dividing line Lfor the third diffraction grating 245 c. The fourth diffraction grating245 d changes the direction of the light path of the light beam passingthe fourth diffraction grating 245 d so that the light beam enters thephotodetector 246 arranged in the opposite side of the dividing line L(or optical axis 4 a) for the fourth diffraction grating 245 d. Thesethird and fourth diffraction gratings 245 c, 245 d correspond to thesecond diffraction grating previously described.

[0142] The fifth diffraction grating 245 e is provided to detect focuserror. The two-divided photodetector 249 is arranged in the same planeas the photodetectors 246, 248. The fifth diffraction grating 245 ecauses the light beam passing the fifth diffraction grating 245 e toenter the two-divided photodetector 249. This fifth diffraction grating245 e corresponds to the third diffraction grating previously described.These first to fifth diffraction gratings 245 a to 245 e have gratingsof different shapes resulting in different stripe patterns.

[0143]FIG. 23 shows a structure of the optical disk apparatus providedwith the optical apparatus shown in FIG. 22. In FIG. 23, the light beamemitted from the light source 7 is converted to a parallel light beam bythe collimator lens 12. The light source 7 formed of a semiconductorlaser is mounted on the bottom surface of the housing of the package240. The optical element 245 is mounted at the position on the housingopposed to the collimator lens 12. A couple of photodetectors 246, 248and two-divided photodetector 249 are fixed on the bottom surface of thehousing in which the semiconductor laser 7 is mounted. Therefore, aplurality of diffraction gratings 245 a to 245 e forming the opticalelement 245 are arranged between the collimator lens 12 corresponding tothe optical system or condenser lens and the photodetector.

[0144] This coherent light beam is condensed by the objective lens 20.The condensed light beam illuminates the disk 9. In FIG. 23, thecondensing optical system 8 is structured by including the collimatorlens 12 and objective lens 20. The beam splitter is not provided in thiscondensing optical system 8. The condensing optical system not includingthe beam splitter is formed in very small size and therefore the opticaldisk apparatus using this condensing optical system 8 can also be formedin small size.

[0145] Deviation between the light beam spot formed by the objectivelens 20 and track on the disk 9 can be detected in the same manner asthe optical apparatus shown in FIG. 20 or the optical disk apparatusshown in FIG. 21. Namely, the optical apparatus structured by the firstand second diffraction gratings 245 a, 245 b provided in the opticalelement 245 and a couple of photodetectors 246, 248 has the samefunction as the optical apparatus shown in FIG. 20. Therefore, adifferential output of a couple of photodetectors 246, 248 becomes thetracking error signal from which offset can be eliminated.

[0146] When the objective lens 20 moves in the direction to be isolatedfrom the disk 9, the light spot on the two-divided photodetector 249moves in the direction to come close to the optical axis 245 z of theoptical element 245 on the two-divided photodetector 249. When theobjective lens 20 moves in the direction to come close to the disk 9,the light spot on the two-divided photodetector 249 moves in thedirection to be isolated from the optical axis 245 z of the opticalelement 245 on the two-divided photodetector 249. Focus error can bedetected by detecting movement of this light spot with the two-dividedphotodetector 249.

[0147] The first and second diffraction gratings 245 a, 245 b formingthe circular region have a diameter which is equal to or a littlesmaller than the beam diameter of the light beam passing the opticalelement 245. When the objective lens 20 is displaced by about 200 μmfrom the center 4 a of the light beam of the objective lens 20, thelight beam is deviated by about 60 μm from the center 4 a of the lightbeam on the optical element 245. Moreover, the radius of the light beamis 475 μm at the areas where the light beam enters the optical element245. In this case, the radius of the circular region is set to 460 μm.Namely, the radius of this circular region is reduced by 15 μm from theradius of the light beam. The diameter of this circular region shouldpreferably be a little smaller than the diameter of the light beam.However, the diameter of this circular region is not required to besmaller than the diameter of the light beam in a degree equal to orlarger than deviation of the light beam. Here, this light beam does notinclude Gaussian distribution. The skirt portion of the Gaussiandistribution of the light beam is cut by the collimator lens 12.

[0148] In this embodiment, the light source, tracking error detector andfocus error detector can be integrally housed in the package, to furtherpromote reduction in size and low price of the optical apparatus.

[0149] In this embodiment, the fifth diffraction grating 245 e is formedin the region where is not easily influenced by change of intensity ofthe light beam due to diffraction of the light. As will be apparent fromFIG. 2, intensity change on the dividing line is smaller than intensitychange in the region isolated fiom the dividing line. Since the fifthdiffraction grating 245 e is integrated to the optical element 245, theintensity of the push-pull signal is rather intensified, to result in astable track error signal.

[0150] In the embodiments of FIGS. 19, 21 and 23, the collimator lenscorresponds to the condenser lens. Moreover, in the embodiments of FIGS.19, 21 and 23, it is of course possible that the optical elements 225,235 and 245 may be provided in the light path through which thereflected light beam divided by the beam splitter 13 passes.

[0151]FIG. 24 shows yet another example of the optical element. In FIG.24, an optical element 255 is provided, at its center area, with acircular region. This circular region includes a couple of regions 255a, 255 b divided by the dividing line. This circular region has adiameter which is equal to or a little smaller than the diameter of thelight beam. These two regions 255 a, 255 b are transmitting regionscausing the light beam to travel in straight. These two regions 255 a,255 b are not provided with diffraction gratings and are onlytransparent flat glass plates. The light beam entering these regions 255a, 255 b runs in straight to radiate the two-divided photodetector (notillustrated) 136. This circular region is provided with a shieldingregion 255 c. This shielding region 255 c is coated with a black paint.The light beam does not pass through this shielding region 255 c. Theoptical element 255 is also provided, at the external side of thecircular region, with the first and second diffraction gratings 255 d,255 e. The first and second diffraction gratings have the same functionsas the diffraction gratings 145 c, 145 d shown in FIG. 7. Therefore, thefirst and second diffraction gratings 255 d, 255 e may be formed of theprism or lens shown in FIG. 14 or FIG. 15.

[0152] The shielding region 255 c shields the part not overlapping theprimary diffracted element among the 0^(th) reflected beam element ofthe light reflected from the disk. As is apparent from FIG. 2, at thecenter area of the light beam where the mirror-reflected light beam andthe primary diffracted light beam do not overlap with each other, theintensity of the light beam almost does not change even if trackdeviation exists. The light beam of this region is not only unnecessaryfor detection of the push-pull signal, but also causes the amplitude ofthe push-pull signal to be reduced.

[0153] The shielding region 255 c increases the amplitude of thepush-pull signal. When the bias element generated in the push-pullsignal is equal owing to optical axis displacement of the objectivelens, since offset of the push-pull signal having a higher amplitudebecomes smaller than that of the push-pull signal having a loweramplitude, this shielding region 255 c is preferable for elimination ofoffset of the push-pull signal.

[0154]FIG. 25 shows still another example of the optical element. InFIG. 25, an optical element 265 is provided with the regions in the samenumber as the optical elements 255 shown in FIG. 24. The five regionsprovided in the optical element 265 are formed in the same shape as thefive regions of the optical element 255. The optical element 265 isprovided, in its center region, with a circular region. This circularregion has a diameter which is equal to or a little smaller than thebeam diameter of the light beam. This circular region includes a coupleof regions 265 a, 265 b divided by the dividing line. This circularregion is provided with a first diffraction grating 265 a and a seconddiffraction grating 265 b. The first and second diffraction gratings 265a, 265 b have the same functions as that of the optical elements shownin FIGS. 18 to 23. Namely, the first and second diffraction gratings 265a, 265 b polarize the optical axis 4 a of the light beam passing thefirst and second diffraction gratings 265 a, 265 b, causing the lightbeam to enter the photodetector located at the position shifted from theoptical axis 4 a. The optical element 265 is provided with the thirddiffraction grating 265 c between the first and second diffractiongratings 265 a, 265 b. In the region where the third diffraction grating265 c is formed, the primary diffracted element of the 0^(th) reflectedelement at the center of the light beam reflected from the disk 9 is notoverlapped. The optical element 265 is provided with the fourth andfifth diffraction gratings 265 d, 265 e at the external regions of thefirst, second and third diffraction gratings 265 a to 265 c. The fourthdiffraction grating 265 d has the same function as the diffractiongratings 215 c, 23 5 c, 245 c shown in FIGS. 18 to 23. The fifthdiffraction grating 265 e has the same function as the diffractiongratings 215 d, 235 d, 245 d shown in FIGS. 18 to 23.

[0155] The third diffraction grating 265 c causes the element of thepart not overlapping on the primary diffracted element among the 0threflected element of the disk reflected light beam to be diffracted tothe position where the photodetector is not provided. Therefore, thisembodiment provides the effect similar to that of the embodiment shownin FIG. 24.

[0156] Moreover, in this embodiment, the light beam emitted from thelight source 7 passes the third diffraction grating 265 c. Since thelight beam emitted from the light source 7 is never shielded, theoptical element 265 can be applied to the optical disk apparatus shownin FIGS. 19, 21 and 23.

[0157]FIG. 26 shows an example of the photodetector. This photodetectorhas a plurality of light receiving sections which receive at least aportion of the light beam for purposes of balancing the distribution oflight in the light receiving sections if the optical axis of the lightbeam is displaced with respect to the light receiving sections when thelight beam is aligned with the target.

[0158] This photodetector 276 is provided with a first light receivingsection 276 a and a second light receiving section 276 b. The firstlight receiving section 276 a is provided with a semi-circular region276 a 1. The second light receiving section 276 b is provided with asemi-circular region 276 b 1. These semi-circular regions 276 a 1, 276 b1 have a diameter which is equal to or a little smaller than the spotdiameter of the light beam. The first light receiving section 276 a isalso provided with the rectangular region 276 a 2 formed at the externalside of the semi-circular region 276 b 1. The second light receivingsection 276 b is provided with the rectangular region 276 b 2 formed atthe external side of the semi-circular region 276 a 1. The first lightreceiving section 276 a physically couples the semi-circular region 276a 1 and rectangular region 276 a 2. Moreover, the second light receivingsection 276 b physically couples the semi-circular region 276 b 1 andrectangular region 276 b 2. The first light receiving section 276 a andsecond light receiving section 276 b are isolated. A differential outputof the first and second light receiving sections 276 a, 276 b is apush-pull signal.

[0159] The photodetector 276 is arranged in such a relation that thedividing line L is aligned in the direction parallel to the track. Thisphotodetector 276 has the function similar to that of the opticalelement explained above. Here, the size of the semi-circular region andrectangular region is set to satisfy the conditions of formulae (2),(3). For example, when the light beam shifts in the direction of arrowmark A (direction orthogonal to the dividing line L), the light beamilluminates a part of the semi-circular region 286 b 1 and the amount ofreceiving light of the second light receiving section 276 b isrelatively reduced in comparison with that of the first light receivingsection 276 a. On the other hand, since the light beam enters thesemi-circular region 276 a 1 and rectangular region 276 b 2, the amountof receiving light of the rectangular region 276 b 2 is added to theamount of receiving light of the semi-circular region 276 b 1.Accordingly, since the amount of receiving light of the second lightreceiving section 276 b increases relatively in comparison with theamount of receiving light of the first light receiving section 276 a,imbalance in the amount of receiving light of the first and second lightreceiving sections 276 a and 276 b is corrected.

[0160] This photodetector 276 corresponds to the optical means, providedbetween the condenser lens and photodetector, for correcting offset.

[0161] The first light receiving section 376 a may be coupledelectrically after the semi-circular region 276 a 1 and rectangularregion 276 a 2 are formed in isolation. Moreover, it is of coursepossible that such structure can be applied to the second lightreceiving section 276 b.

[0162]FIG. 27 shows a practical example of the optical apparatus shownin FIG. 23. As shown in FIG. 27, the light source 7, a couple ofphotodetectors 246, 248 and two-divided photodetector 249 are mounted onthe same silicon substrate 247. Namely, the tracking error detector andfocus error detector are mounted on a single silicon substrate 247utilizing ordinary integrated circuit producing technology. In thisstructure, since all servo detectors may be mounted by merely mountingthe silicon substrate 247 to the housing of package 240, the number ofparts and assembling procedures call remarkably be reduced.

[0163] The position of the optical element 245 is not required to beadjusted on the package 240. The optical element 245 can be located tothe predetermined position through the positional adjustment using acouple of photodetectors 246, 248 for tracking error detection and thepositional adjustment using the two-divided photodetector 249 for focuserror detection, because the optical element 245 is mounted in thepredetermined positional relationship on the same package 240 with thephotodetectors 246, 248, 249.

[0164] On the silicon substrate 247, the electrodes 246 z, 248 z, 249 z1, and 249 z 2 of the detectors are formed. The electrode of the lightsource 7 is not illustrated In FIG. 27.

[0165] The embodiment explained above relates to the case where axialdisplacement is generated due to a shift of the objective lens, butoffset can also be corrected even if axial deviation is generated due toan assembling error.

[0166] Moreover, if axial displacement of the optical beam is generateddue to inclination, imbalance in the amount of receiving light can becorrected by providing an optical means between the condenser lens andphotodetector.

[0167] In addition, in this embodiment, the diameter of the centerregion of the optical means is set, for explanation, equal to or alittle smaller than the beam diameter of the light beam passing theoptical means. When the diameter of the center area is smaller than thebeam diameter, positional adjustment of the optical means can be doneeasily. This is because since a part of the external circumference ofthe light beam is set to the size as passing the diffraction gratingregion, imbalance in the amounts of receiving light of a couple of lightreceiving sections can be absorbed by the effect of the diffractiongrating region, so some optical means mounting error is allowed.

[0168] In addition, the optical element may be arranged, in the presentinvention, before the condenser lens. Namely, the optical element may bearranged between the objective lens and condenser lens. In this case,the diffraction grating formed to the optical element is defined by theamount of diffraction depending on the arrangement position.

[0169] Moreover, in the present invention, a plurality of diffractiongratings (for example, diffraction gratings 245 a to 245 e shown in FIG.27) may be formed directly on the condenser lens. When one surface ofthe condenser lens is flat, such diffraction grating may be easilyformed by attachment to the flat surface of the condenser lens. Namely,the diffraction grating can be formed by coating the plastic material onthe flat surface and then forming a slit to the plastic material.

[0170] This embodiment has been described mainly in its optical systemfor tracking error detection but the present invention can also beapplied to an optical system for focus error detection and an opticalsystem for an information reading.

[0171] The present invention has been described with reference toparticular embodiments thereof but it should be understood that theembodiments described are only examples for application of the principleof the present invention. For instance, the optical apparatus can beapplied not only to the magneto-optical disk apparatus but also to alloptical disk apparatuses. In addition, this optical apparatus can beapplied not only to an optical disk apparatus but also to all opticalapparatuses for detecting focus error and tracking error.

[0172] The many advantages of the invention are now apparent. Imbalancein the amount of receiving light (offset) by the axial deviation of thelight beam resulting from axial displacement of the objective lens,inclination of the target object or assembling error of the opticalsystem can be reduced. Offset resulting from assembling errors can alsobe eliminated. Moreover, since the optical means is arranged between thecondenser lens and photodetector, the photodetector in any opticalsystem can correct the offset, and the condenser lens and photodetector,the condenser lens can be arranged within the necessary region andoffset can also be eliminated with a simplified and low cost structure.High precision, high speed tracking and highly accurate informationreading can be conducted.

[0173] Various embodiments of the invention have additional advantages.The beam splitter and condenser lens for detecting tracking error are nolonger required. Since the space between the condenser lens and lightreceiving section is used, an additional region is not required forarranging the optical means. Moreover, the tracking error detector canbe mounted easily. Optical elements such as a beam splitter andcondenser lens are not required to detect focus error. Also, the focuserror detector can be installed easily, and a plurality of opticalelements can be structured more easily.

[0174] While the principles of the invention have been described abovein connection with specific apparatus and applications, it is to beunderstood that this description is made only by way of example and notas a limitation on the scope of the invention.

What is claimed is:
 1. An optical apparatus for decoding information stored in a target comprising a source of light, a condenser lens through which the light passes, the condenser lens forming the light into a beam which has an optical axis, the beam being sent to the target after the light passes through said condenser lens, the target encoding the light beam with the information and directing the encoded light beam to an optical detector, said optical detector having a plurality of light receiving sections, each of which can receive at least a portion of the encoded light beam for purposes of aligning the light beam with the target, and means for balancing the distribution of light in said light receiving sections if the optical axis of the light beam is displaced with respect to the light receiving sections when the light beam is aligned with the target.
 2. The optical apparatus of claim 1wherein said balancing means comprises an optical device through which the light beam passes before it is received by said optical detector, said optical detector having a circular-like region through which the light passes without substantial interference, said circular-like region having a diameter which is equal to or smaller than the diameter of said light beam, said circular-like region further having at least first and second halves, wherein light passing through said first half is directed towards a first said light receiving section, and light which passes through said second half is directed to a second said light receiving section, said optical device further having a first light directing device outside of said first half of said circular-like region which directs light to said second light receiving section, and a second light directing device outside of said second half of said circular-like region which directs light to said first light receiving section.
 3. The optical apparatus of claim 2 wherein said circular-like region is circular.
 4. The optical apparatus of claim 2 wherein said first and second circular-like regions are formed of arcs having a radius which is greater than half the total width of said circular-like region, and an origin displaced from the center of said optical device.
 5. The optical apparatus of claim 2 wherein said circular-like regions form an elliptical shape.
 6. The optical apparatus of claim 2 wherein said first and second light directing devices are diffraction gratings.
 7. The optical apparatus of claim 2 wherein said first and second light directing devices are prisms.
 8. The optical apparatus of claim 2 wherein said first and second light directing devices are circular lenses.
 9. The optical apparatus of claim 2 wherein said first and second halves are third and fourth light directing devices, said first and second halves of said optical detector being offset from said axis on different sides of said optical device.
 10. The optical apparatus of claim 9 comprising a second optical detector, said optical device further having a fifth light directing device which directs part of said light beam to said second optical detector.
 11. The optical apparatus of claim 10 wherein said third optical detector is offset with respect to said axis.
 12. The optical apparatus of claim 11 wherein said light source, said optical detector and said second optical detector are mounted on a single substrate.
 13. The optical apparatus of claim 11 wherein the target is a track in a track medium, said optical detector generates a tracking error signal, and said second optical detector generates a focus error signal.
 14. The optical apparatus of claim 10 wherein said first, second, third, fourth and fifth are formed as a single thin plate type diffraction grating.
 15. The optical apparatus of claim 2 comprising a shielding region in the circular-like regions.
 16. The optical apparatus of claim 15 wherein said shielding region is opaque.
 17. The optical apparatus of claim 15 wherein said shielding region is a diffraction grating.
 18. The optical apparatus of claim 9 wherein said light source and said optical detector are mounted on a single substrate.
 19. The optical apparatus of claim 1 wherein said balancing means comprises an optical device through which the light beam passes before it is received by said optical detector, said optical device having a circular-like region through which the light passes, said circular-like region having a cross-sectional area which is equal to or smaller than the cross-sectional area of the light beam, said circular-like region further having at least first and second halves, wherein light passing through said first half is directed towards a first said light receiving section, and light which passes through said second half is directed to a second said light receiving section, the paths of the light passing through said first and second halves crossing each other as they approach said optical detector, said optical device further having a first light directing device outside of said first half of said circular-like region which directs light to said second light receiving section, and a second light directing device outside of said second half of said circular-like region which directs light to said first light receiving section, the paths of the light passing through said first and second light directing devices not crossing each other as they approach said optical detector.
 20. The optical apparatus of claim 19 wherein said first and said second halves of said circular-like region are conical prisms.
 21. The optical apparatus of claim 19 wherein said first and said second halves of said circular-like region are third and fourth diffraction gratings.
 22. The optical apparatus of claim 21 wherein said optical detector is offset with respect to said optical axis.
 23. The optical apparatus of claim 1 wherein the target is a track in a disk medium.
 24. An optical apparatus for decoding information stored in a target comprising a source of light, a condenser lens through which the light passes, the condenser lens forming the light into a beam which has an optical axis, the light beam being sent to the target after the light beam passes through said condenser lens, the target encoding the light beam with the information and directing the encoded light beam to an optical detector, said optical detector having a plurality of light receiving sections to receive at least a portion of the encoded light for purposes of balancing the distribution of light in said light receiving sections if the optical axis of the light beam is displaced with respect to the light receiving sections when the light beam is aligned with the target.
 25. The optical apparatus of claim 24 wherein said optical detector includes a first light receiving section provided with a first semicircular-like region, a second light receiving section provided with a second semicircular-like region, the first and second semicircular regions having a diameter equal to or smaller than the diameter of said light beam, a third light receiving section formed adjacent the external side of said second light receiving section, and a fourth light receiving section formed adjacent the external side of said first light receiving section, said first and third light receiving sections being coupled so that their respective outputs are added together, said second and fourth light receiving section being coupled so their respective outputs are added together, whereby the distribution of light in said light receiving sections is balanced if the optical axis of the light beam is displaced with respect to the light receiving sections when the light beam is aligned with the target. 